Conventionally, a lubricating oil has been used for an internal combustion engine, automatic transmission, grease and the like in order to operate them smoothly. Among these uses, the lubricating oil for an internal combustion engine (sometimes called an “engine oil”) is particularly required to exhibit a high performance due to the high performance, high power, and severe operating conditions of the internal combustion engine.
In order to achieve the above required performance, various additives such as an anti-wear agent, metallic detergent, ashless dispersant and antioxidant are contained in the conventional lubricating oil for an internal combustion engine. Above all, zinc dialkyl dithiophosphate (ZnDTP) is used in the lubricating oil for an internal combustion engine as an essential additive since it can serve as the anti-wear agent and antioxidant (see below Patent Document 1 for example).